body {
  background-image: url(http://api.xiaohigh.com/image/random);
  background-size: cover;
  .box:first-child {
    //向上取整
    // width: ceil(3.1) + 0px;
    //向下取整
    // width: floor(4.9);
    //百分比
    // width: percentage((1/6));

    width: 500px;
    height: 100px;
    // background: darken(hsl(192, 60%, 44%), 10%);
    background: fadein(hsla(192, 60%, 44%, 50%), 10%);
  }

  .box:nth-child(2) {
    width: 500px;
    height: 100px;
    // background: darken(hsl(192, 60%, 44%), 20%);
    background: fadein(hsla(192, 60%, 44%, 50%), 20%);

  }

  .box:nth-child(3) {
    width: 500px;
    height: 100px;
    // background: darken(hsl(192, 60%, 44%), 30%);
    background: fadein(hsla(192, 60%, 44%, 50%), 30%);

  }
  .box:nth-child(4) {
    width: 500px;
    height: 100px;
    // background: darken(hsl(192, 60%, 44%), 40%);
    background: fadein(hsla(192, 60%, 44%, 50%), 40%);

  }
}